1. Field of the Invention
The present invention relates to a method of digitally controlling a magnetic bearing spindle and a control apparatus therefor. The invention relates more specifically to a method of digitally controlling each bearing of a multiaxis control type magnetic bearing spindle and a control apparatus therefor.
2. Description of the Related Art
FIG. 5 is a perspective view showing the appearance of a conventional five-axis control type magnetic bearing spindle and a schematic block diagram of a control system.
The five-axis magnetic bearing spindle will now be described as an example of a multiaxis type magnetic bearing spindle with reference to FIG. 5. One end of a spindle 2 is radially supported by a radial magnetic bearing 10 having electromagnets for X and Y axes 11X and 11Y as well as electromagnets for X and Y axes 12X and 12Y, which are mutually arranged at right angles on an XY plane perpendicular to the axis of spindle 2. Similarly, the other end of spindle 2 is radially supported by a radial magnetic bearing 20 having electromagnets for X and Y axes 21X and 21Y as well as electromagnets for X and Y axes 22X and 22Y. Spindle 2 is driven by a motor 9 to rotate in a direction indicated by an arrow B. Further, spindle 2 is axially supported by a thrust magnetic bearing 8 to avoid axial (in the Z direction) displacement.
On the X and Y axes on the plane where the above mentioned radial magnetic bearing 10 is arranged, positioned are position sensor coils for X and Y axes 14X and 14Y, which detect displacement in the directions of X and Y axes with respect to the reference position in spindle 2, and generate outputs in accordance with the amount of displacement. Similarly, position sensor coils for X and Y axes 24X and 24Y are positioned in X and Y axes on the plane where radial magnetic bearing 20 is arranged.
Further, provided on the end surface at one end of spindle 2 is a sensor 7 for detecting axial displacement, which is connected to a control circuit 30 along with position sensor coils for X and Y axes 14X and 14Y as well as position sensor coils for X and Y axes 24X and 24Y.
Electromagnets for X and Y axes 11X and 11Y respectively include control windings 13X and 13Y for adjusting electromagnetic force, which are controlled by control circuit 30. Similarly, electromagnets for X and Y axes 21X and 21Y respectively include control windings 23X and 23Y for adjusting electromagnetic force, whose exciting current is controlled by control circuit 30. Thrust magnetic bearing 8 is also controlled by control circuit 30.
FIG. 6 is a block diagram of the control circuit shown in FIG. 5. In FIG. 6, outputs from the sensors in the axes shown in FIG. 5 are respectively applied to A/D converters 31 to 35 as input signals for first to fifth channels to be sampled, converted to digital signals and applied to an MPU 36. MPU 36 performs control operation for five axes and applies the results to D/A converters 37 to 41. D/A converters 37 to 41 convert control signals to analog signals to output analog signals for every axis. Each analog control signal is amplified by an amplifier, not shown, and applied to the electromagnet for the corresponding bearing.
FIG. 7 shows a process flow of MPU 36 during one sampling period. In MPU 36 shown in FIG. 6, control operations for the A/D converted digital signals from 1 to 5 channels are sequentially performed, and the operation results are then converted to analog signals by D/A converters 37 to 41 at the same time. Here, the control operation includes, for example, proportional operation, integral operation, phase compensation operation, gyro compensation operation or the like.
In the conventional example shown in FIG. 6, each axis requires one of A/D converters 31 to 35, which are relatively expensive electronic parts, increasing cost for the apparatus as a whole. In addition, every factor of control operation, such as proportional, integral, phase compensation and gyro compensation operations, is performed in the same sampling period. The sampling frequency is therefore determined such that phase compensation operation for high frequency component is not affected by phase lag due to dead time. Commonly, sampling frequency is set on the side of at least one decade higher frequency in control frequency band, or it is determined from the time required for control operation, often governing control band. It is therefore of great importance how to reduce operation time to achieve higher controllability.
However, high-speed sampling is not necessarily required for every control operation and operations such as proportional, integral and gyro compensation operations are directed for a relatively low frequency component, so that even the slowest sampling frequency is sufficient.